Hitherto, leakage gas has been detected by bringing sucked gas into direct contact with a sensor portion and measuring gas concentration based on a change in value of electrical resistance or current. However, such a known gas detector is of the sensor type that an area capable of being monitored by one detector is narrow and leakage gas cannot be detected unless the gas reaches the detector. Accordingly, there has been a risk that, in the event of a gas leakage, an alarm error may occur depending on the direction of wind and the position where the detector is installed. Another problem is that, in a gas refinery or the like, a very large number of gas detectors must be installed and a substantial cost is required (see Patent Reference 1).
On the other hand, to solve the above-mentioned problem, a gas visualizing device for remotely monitoring the presence of a gas leakage has been proposed. Such a gas visualizing device employs a laser beam source for irradiating an infrared laser beam having the same wavelength as the absorption wavelength of gas to be measured, and the absorption of an infrared ray, which is reflected from the background, is imaged using an image sensor to be displayed in the form of a two-dimensional visible image.
However, that known gas visualizing device requires a very large-sized and high-power laser beam source and therefore has a serious problem in point of cost. Another problem is that the displayed two-dimensional image is greatly affected by weather conditions and temperatures, and a difficulty arises in discriminating the occurrence of a gas leakage from shinning of sunlight. For those reasons, the known gas visualizing device has not been suitable for monitoring a gas leakage in practical fields (see Patent Reference 2).
Further, in the case of hydrogen gas, in spite of being an energy medium with a high risk in such a point as causing explosion if ignited, the hydrogen gas is tasteless, colorless, and odorless. In addition, even if ignited, a flame of hydrogen gas is substantially transparent and invisible to the naked eye under sunlight. Meanwhile, several publications disclose detection techniques of selecting ultraviolet light generated upon the occurrence of corona discharge by an interference filter, collecting an ultraviolet ray having passed through the interference filter, and visualizing the collected ultraviolet ray in the form of a visible image using an ultraviolet image tube or a TV camera (see Patent References 3 and 4).
Patent Reference 1Japanese Patent Laid-open No. 6-307967Patent Reference 2Japanese Patent Laid-open No. 6-288858Patent Reference 3Japanese Patent Publication No. 5-40874Patent Reference 4Japanese Utility Model Laid-open No. 61-174680